To the east of Ben Cruachan it is highly likely that the weather was perfectly reasonable VFR. In the lee of that ridge running down the east of Loch Linnhe it would have had a very low ceiling. To the west more typical of the Tiree conditions
Over Tiree where there is a single area of high ground and the rest of the island is about 20ft high it would have been reasonable to descend over the sea and sneak in. These conditions are not unusual on the west coast and it is possible the pilot was familiar with them - or equally they were not. But being natives of the island and IMC rated? Likely they were familiar with the typical conditions.